In recent years, in WDM (Wavelength Division Multiplex) optical transmission systems, metro core systems capable of interconnecting local hub cities and inserting (adding) or branching (dropping) an optical signal of an arbitrary wavelength at arbitrary nodes are becoming a focus of attention. FIG. 8 illustrates an overview of such a system.
The WDM optical transmission system shown in FIG. 8 is configured by connecting a plurality of OADM (Optical Add-Drop Multiplexer) nodes in a ring shape via a transmission path (optical fiber). The OADM at each OADM node is designed to allow light (channel) of an arbitrary wavelength to be inserted into the transmission path (node A and node B in FIG. 8) and allow light of an arbitrary wavelength out of transmitted WDM signal light to be branched (node C) from the transmission path. The OADM, configured as illustrated in FIG. 8, is an apparatus that demultiplexes WDM signal light into channels through an optical multiplexer/demultiplexer using an AWG (Arrayed Waveguide Grating) or the like and inserts or branches the optical signal channel by channel using an optical switch. The OADM can provide a variable optical attenuator (VOA) as optical power adjusting section for each channel of the WDM signal light. The variable optical attenuator provided channel by channel is designed to be able to compensate for a power variation for each wavelength (channel by channel) and perform automatic level control (ALC).
Each OADM node is provided with an optical amplifier (AMP) for received light and an optical amplifier for transmission light before and after the OADM respectively. The front-end optical amplifier compensates for signal light loss produced in the transmission path between the own node and an upstream node. On the other hand, the back-end optical amplifier compensates for signal light loss produced in the OADM or the like within the own node. These optical amplifiers are intended to extend a transmission distance. These optical amplifiers using an EDFA (Erbium Doped Fiber Amplifier) or the like perform automatic gain control (AGC).
In such a WDM optical transmission system, malfunctions during system operations, such as a disconnection of a connector provided in the transmission path and disconnection of a fiber or the like, need to be taken into consideration. An example of this malfunction is illustrated in FIG. 9. In the illustrated example, a communication malfunction has occurred between node A and node B, preventing an optical signal inserted at node A from being received at node B. As a result, there is no more optical signal corresponding to the wavelength inserted at node A from the malfunction span (between node A and node B) onward, which causes the number of wavelengths of the WDM signal light, to change over a short span of time.
For example, suppose the malfunction causes the number of wavelengths of the WDM signal light to drastically reduce from 10 to 1. There is a period during which the response of ALC cannot catch up with the drastic change in the number of wavelengths and ALC corresponding to WDM signal light for 10 waves before the malfunction continues in this transient period although the remaining one wave of WDM signal light is actually transmitted. That is, during the transient period, transient ALC that controls the power of the remaining one wave to power corresponding to 10 waves is performed.
In this case, a variation occurs in the signal light power according to the remaining wavelength. One main cause is stimulated Raman scattering that occurs in a transmission path, dispersion compensating module (DCM) or the like.
Stimulated Raman scattering (SRS) is a phenomenon that occurs in an optical fiber or dispersion compensating fiber (DCF) used in the transmission path. The SRS is a phenomenon that when WDM signal light propagates in the transmission path, part of the light power on the short wavelength side contributes as excitation light and the light power on the long wavelength side is thereby amplified (a Raman amplifier uses this phenomenon). In a situation in which SRS has occurred, when light on the short wavelength side disappears due to the occurrence of a malfunction and the light on the long wavelength side under the influence of the SRS by the light on the short wavelength side remains, the influence of the SRS disappears, and as a result, the power of the residual light falls below the average power before the occurrence of the malfunction. On the contrary, when the light on the long wavelength side disappears due to the occurrence of the malfunction and the light on the short wavelength side remains, the portion that can be allocated to excitation light disappears, and as a result, the power of the residual light exceeds the average power before the occurrence of the malfunction. That is, the effect of the SRS changes according to the number of signal wavelengths and the location of WDM signal light propagating through the transmission path and a variation occurs in the power of WDM signal light before and after the occurrence of the malfunction.
When the number of wavelengths (number of channels) of the WDM signal light drastically changes, the output wavelength characteristics of the optical amplifier changes caused by the SRS and the output light power of the residual light (remaining channel) changes. Even if the amount of variation at one node is not large, if the above described transient ALC is executed at each node through which the signal light passes and there are many optical amplifiers that execute AGC, power variations are accumulated as the number of nodes through which the signal light passes, that is, the number of relays increases.
In an example shown in FIG. 10, when this accumulation of power variations exceeds an allowable range of reception (input dynamic range) of the light receiving apparatus, good reception characteristics can no longer be obtained and the number of errors in the residual light may increase.
Table 1 below illustrates a relationship between a variation in the signal light power and wavelength condition when the number of wavelengths decreases. The following cases can be assumed as patterns of decrease in the number of wavelengths.
1. When the wavelength (signal wavelength) of the residual light corresponds to the short wavelength side. 2. When the wavelength corresponds to the central waveband. 3. When the wavelength corresponds to the long wavelength side. 4. When the wavelength is divided into the short wavelength side and long wavelength side.
TABLE 1Signal wavelength (residual light)After changeSignal lightWavelengthIn normalof numberpowerCauseconditionoperationof wavelengthsvariationSRS1AllOne wavelengthPlus◯wavelengths(short wavelengthside)2AllOne wavelengthSmallΔwavelengths(central waveband)3AllOne wavelengthLarge minus◯wavelengths(long wavelengthside)4AllSeveral wavelengthSmallΔwavelengths(both wavelengthsides)
In the case of the wavelength condition 1 where the wavelength of the residual light corresponds to the short wavelength side, when, for example, all wavelengths of transmittable signal band are being transmitted, if one wavelength on the short wavelength side remains, the residual light is susceptible to a power variation under the influence of SRS and the power variation direction observed in the output light of the optical amplifier relatively increases on the plus side. In the light receiving apparatus under the control of the node provided in the transmission path downstream of the location of a malfunction that causes a decrease in the number of wavelengths, power variations are accumulated as the number of relays increases, and therefore the power of the signal light inputted to the light receiving apparatus may exceed an allowable range of reception, thus affecting the signal quality.
On the contrary, in the case of the wavelength condition 3 where the wavelength of the residual light corresponds to the long wavelength side, when, for example, all wavelengths of the transmittable signal band are being transmitted, if one wavelength on the long wavelength side remains, the residual light is susceptible to a power variation under the influence of SRS and the power variation direction observed in the output light of the optical amplifier relatively increases on the minus side. In the light receiving apparatus under the control of the node provided in the transmission path downstream of the location of a malfunction causing a decrease in the number of wavelengths, power variations are accumulated as the number of relays increases, and therefore the power of the signal light inputted to the light receiving apparatus may fall below the allowable range of reception, thus affecting the signal quality.
Furthermore, in the case of the wavelength condition 2 where the wavelength of the residual light corresponds to the central waveband and in the case of the wavelength condition 4 where the wavelength of the residual light is distributed divided into the short wavelength side and the long wavelength side, the power variations are smaller than those in the above described cases.
Regarding a power variation caused by insertion or branching of an optical signal performed according to a normal operating procedure in normal operation where no malfunction has occurred, since the duration of variation in the number of wavelengths (variation speed) is scheduled, ALC that compensates for a power variation for each wavelength is possible using a variable optical attenuator provided for the OADM channel by channel as light power adjusting section as described above. However, in cases caused by a malfunction such as a fiber disconnection, the speed of variation in the number of wavelengths is unscheduled and the variation duration is short, and therefore a power variation occurs at higher speed than the response speed (response time) of the variable optical attenuator and the response speed of output control of the optical amplifier and an error may occur in the light receiving apparatus. Therefore, the problem in avoiding errors in the light receiving apparatus is how to compensate for a high-speed power variation. Therefore, there is proposed means for compensating for a light power variation as disclosed in Japanese Patent Laid-Open No. 2006-295113.
The method disclosed in the above described document divides and monitors a signal band and compensates for a power variation in each of the divided bands and is a technique that adjusts a variable optical attenuator by calculating respective power variations of the divided bands. However, such a method for compensating light power variations leads to an increase in complexity of all of the optical circuit, control circuit and calculations, and still has room for improvement in terms of the speed of compensation for power variations and cost.